The invention relates to a potentiometer having a pair of magnetic resistance effect elements cooperating with a permanent magnet carried by a rotary shaft, and more particularly, to an improvement of a bearing holder for the rotary shaft.
A potentiometer of the kind described is used to produce a voltage signal which is a function of the angle of rotation of a system experiencing a rotational motion. The system includes its own rotary shaft, the free end of which carries a permanent magnet of solid semi-cylindrical form, for example. As the magnet moves angularly together with the shaft, the magnetic field created thereby scans alternately the sensor surface of a pair of magnetic resistance effect elements which are disposed in opposing relationship with the free end of the shaft with a clearance therebetween. If the individual sensor surfaces are of an arcuate form of an equal width and are arranged annularly about the axis of the shaft so as to form a circular band, a change which occurs in the area of the sensor surfaces of the elements exposed to the magnetic field will be proportional to a change in the angle of rotation of the shaft. Hence, a proportional change in the resistance of the respective elements will result. The pair of elements are connected in series across a d.c. source, and the potential at the junction between the elements will vary proportionally as the resistance of the elements changes, thus producing an output which represents a linear function of the angle of rotation of the shaft.
When an oscillation is imparted to the shaft of the potentiometer during its angular movement, the area of the elements exposed to the magnetic field of the magnet which is carried by the shaft is influenced thereby, causing a distortion in the output response of the elements. It is thus seen that the shaft must be machined with a high accuracy and properly positioned and supported by precision bearings in order to assure a selected output response. In the prior art practice, at least a pair of precision ball bearings are axially spaced within a holder for supporting the shaft. The bearings represent a large proportion of the product cost and an increase in the number of assembling steps, thus preventing the provision of inexpensive products. An attempt has been made to avoid this difficulty by providing a cylindrical holder formed of a synthetic resin having abrasion and heat resistance and which directly supports the shaft of the potentiometer without the interposition of the ball bearings. However, the attempt resulted in a failure because of the wall thickness on the order of at least 5 mm of the holder which is required to achieve a desired strength, but which caused a distortion when the resin is set during the molding process to produce unevenness of the internal surface or an inner diameter at the both ends which is greater than the inner diameter at an intermediate position along the length.